<html>
<head>
</head>
<body>
  <script>
    var ARROW_ARG = /^([^\(]+?)=>/;
    
    var fn = foo => { foo(); }
    
    var args = fn.toString().match(ARROW_ARG);
    console.log(args); // ["foo =>", "foo ", index: 0, input: "foo => { foo(); }"]
    var args2 = fn.toString().match(ARROW_ARG);
    console.log(args2); // null in Chrome 50
  </script>
</body>
</html>